Light-diffusing optical elements having cladding with scattering centers

ABSTRACT

A light-diffusing optical element with efficient coupling to light sources with high numerical aperture. The light-diffusing optical element includes a higher index core surrounded by a lower index cladding. The cladding includes scattering centers that scatter evanescent light entering the cladding from the core. The scattered light exits the element to provide broad-area illumination along the element. Scattering centers include dopants, nanoparticles and/or internal voids. The core may also include scattering centers. The core is glass and the cladding may be glass or a polymer. The element features high numerical aperture and high scattering efficiency.

This application is a continuation application of U.S. patentapplication Ser. No. 15/838,729 filed on Dec. 12, 2017 and claims thebenefit of priority under 35 U.S.C. § 120 of U.S. patent applicationSer. No. 14/995,570, filed on Jan. 14, 2016, which claims the benefit ofpriority under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No.62/112,852 filed on Feb. 6, 2015 the contents of which are relied uponand incorporated herein by reference in their entirety.

FIELD

The present disclosure relates to optical elements for diffusing lightto provide broad area illumination. More particularly, the presentdisclosure relates to optical elements having high scattering efficiencyto achieve efficient diffusion of light. The optical elements may alsohave high numerical aperture to permit efficient coupling to LED andlaser diode illumination sources.

TECHNICAL BACKGROUND

Many optical systems utilize optical fibers to transmit light from aremote light source to a target destination. In a typical system, thelight source is coupled to the fiber and light supplied by the source isguided by the fiber to the target destination. Optical fibers have beenwidely used in telecommunications to deliver information encoded in theform of an optical signal. A telecommunication link includes atransmitter that converts an electrical signal to an optical signal. Theoptical signal is launched into the fiber and transmitted to a receiverthat reconverts the optical signal back to an electrical signal forfurther processing at the destination end of the link. Optical fibershave also been used as point illumination sources. In theseapplications, light from a source is coupled to the receiving end of thefiber and emerges from the destination end of the fiber as anilluminating beam.

There has recently been interest in extending the use of optical fibersto applications in broad-area illumination. In these systems, theobjective is to achieve controlled release of light along at leastportions of the length of the fiber. Instead of using the fiber toconfine light and transmit it with minimal losses from a source toprovide an optical signal or point illumination to a target positionedin the direction of the fiber axis, the objective is to use the lateralsurface of the fiber as a broad-area source of illumination thatoperates in the radial direction of the fiber.

Light-diffusing fibers are a class of fibers that can be used as abroad-area illumination source. Light-diffusing fibers are designed toscatter light propagating in the direction of the fiber axis in radialdirections out of the fiber. Radial scattering is typically accomplishedby incorporating nanostructural voids in the core region of the fiber.The voids are low-index regions, typically filled with a gas, and havedimensions on the order of the wavelength of the light propagatingthrough the fiber. The refractive index contrast between the voids andsurround dense glass matrix effects scattering of the light. Thescattering efficiency, and hence intensity of scattered light, can becontrolled by controlling the dimensions, spatial arrangement and numberdensity of voids. In addition to broad-area illumination,light-diffusing fibers can be employed in displays and as light sourcesin photochemical applications. Further information about light-diffusingfibers and representative applications can be found in U.S. Pat. Nos.7,450,806 and 8,591,087, the disclosures of which are herebyincorporated by reference herein.

With the increasing trend away from conventional incandescent lightsources, LEDs and laser diodes are become increasingly important lightsources for optical fibers. Efficient coupling of LEDs and laser diodesto optical fibers presents challenges because of mismatches incross-sectional area and numerical aperture (NA). The cross-sectionalareas and numerical apertures of LEDs and laser diodes are much greaterthan the cross-sectional areas and numerical apertures of typicaloptical fibers.

One strategy for improving coupling efficiency is to increase thediameter of the optical fiber. The drawback to this approach, however,is that in order to maintain the flexibility of the fiber, it isdesirable to maintain the diameter of the glass portion (core+cladding)at or below ˜125 μm. Since much higher diameters are needed forefficient coupling to LEDs and laser diodes, this approach has limitedeffectiveness in applications where fiber flexibility is desired.

A second strategy for improving coupling efficiency is to increase thenumerical aperture of the fiber. This strategy, however, is difficult toimplement for conventional light diffusing optical fibers because thevoids used in the core region to provide the scattering efficiencyrequired for light diffusing fibers are low index regions and lead to adecrease in the average refractive index of the core. Since highnumerical aperture is favored by increasing the refractive index of thecore, the need to increase scattering efficiency in light diffusingoptical fibers by including low index voids in the core region conflictswith the goal of increasing the numerical aperture of light diffusingfibers and makes it more difficult to achieve efficient coupling betweenLEDs and laser diodes and light diffusing optical fibers.

There is a need for light diffusing optical elements that coupleefficiently to LEDs and laser diodes while maintaining the flexibilityneeded for deployment in tight space, bent configurations and areaswhere it is impossible to deploy conventional light sources.

SUMMARY

This disclosure provides a light-diffusing element for broad-areaillumination that couples efficiently to LED and laser diode lightsources. The light-diffusing element includes a core and a cladding. Thecore is configured from glass. The cladding is configured from glass ora polymer. The cladding includes scattering centers. The core may alsoinclude scattering centers. Scattering centers include dopants,nanoparticles, and internal voids. Light supplied from the light sourceenters the core and is guided through the light diffusing element.

The scattering centers may be nanostructural or microstructural regionsthat act to redirect light propagating in the direction of the centralaxis of the element in a radial, transverse or off-axis direction. Thescattered light may exit the lateral surface of the element to providebroad-area illumination. Evanescent light entering the cladding from thecore is scattered by scattering centers in the cladding to provide anillumination effect.

The scattering centers may be distributed throughout the cross-sectionaldirection of the element or localized to particular regions thereof. Thescattering centers may be distributed throughout the cross-sectionalarea of the core or localized to particular regions of the core. Thescattering centers may be distributed throughout the cross-sectionalarea of the cladding or localized to particular regions of the cladding.The scattering centers may be in the core, in the cladding, or in bothcore and cladding.

The scattering centers may be configured to scatter one or morewavelengths of light in the range from 25 nm to 20 μm.

The light-diffusing element may couple directly to a light source orcouple to a light source through an intervening element. The lightsource may be a lamp, a laser, a laser diode, or an LED. The interveningelement may be an optic, a clear glass rod, or a light-transmissivepolymer.

The present description extends to:

A light-diffusing element comprising:

a glass core, said glass core having a diameter greater than 65 μm; and

a cladding surrounding said glass core, said cladding having a lowerrefractive index than said glass core, said cladding including firstscattering centers, said first scattering centers having a cross-sectionwith a dimension of at least 25 nm;

wherein said element exhibits light scattering losses of at least 0.1dB/m.

The present description extends to:

An illumination system comprising:

a light source optically coupled to a light-diffusing element, saidlight-diffusing element comprising:

a glass core, said glass core having a diameter greater than 65 μm; and

a cladding surrounding said glass core, said cladding having a lowerrefractive index than said glass core, said cladding including firstscattering centers, said first scattering centers having a cross-sectionwith a dimension of at least 25 nm;

wherein said element exhibits light scattering losses of at least 0.1dB/m

The present description extends to:

A method for forming a light-diffusing element comprising:

forming a core, said core comprising glass;

forming a cladding on said core, said cladding including firstscattering centers, said first scattering centers having a cross-sectionwith a dimension of at least 25 nm.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments as described herein, including the detailed descriptionwhich follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understanding the natureand character of the claims. The accompanying drawings are included toprovide a further understanding, and are incorporated in and constitutea part of this specification. The drawings illustrate one or moreembodiment(s), and together with the description serve to explainprinciples and operation of the various embodiments.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the description serve to explain principles and operationof the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a cross-section of a light-diffusingelement.

FIG. 2 is a schematic depiction of a cross-section of a light diffusingelement having scattering centers in the core.

FIG. 3 is a schematic depiction of a cross-section of a light diffusingelement having scattering centers in the cladding.

FIG. 4 is a schematic depiction of a cross-section of a light diffusingelement having scattering centers in the core and cladding.

FIG. 5 illustrates illuminations systems that include a light sourcecoupled to a light-diffusing element.

FIG. 6 illustrates a light-diffusing element having a glass core, apolymer cladding and protective coating.

FIG. 7 compares the calculated attenuation of an optical signal inlight-diffusing elements having scattering centers in the form ofinternal voids and nanoparticles.

DETAILED DESCRIPTION

The present disclosure provides a light-diffusing element for broad-areaillumination that couples efficiently to a variety of light sources,includes LEDs and laser diodes. The light-diffusing element includes acore and a cladding. The core is configured from glass. The cladding isconfigured from glass or a polymer. The cladding includes scatteringcenters. The core may also include scattering centers. Scatteringcenters include dopants, nanoparticles, and voids. Light supplied fromthe light source enters the core and is guided through the lightdiffusing element. The direction of propagation of the guided light inthe light-diffusing element may be referred to herein as thelongitudinal or axial direction. The scattering centers effectscattering of light propagating in the light-diffusing element. As usedherein, scattering refers to a redirection of the propagating light in adirection other than the longitudinal direction. The direction ofscattered light may be referred to as an off-axis direction, a lateraldirection, or a transverse direction. At least a portion of thescattered light passes the lateral surface of the light-diffusingelement and exits the light-diffusing element to provide an illuminationeffect. The illumination effect may be broad-array illuminationprovided, for example, by illumination along all or a portion of thelength of the element.

The light-diffusing element includes a core and a cladding surroundingthe core. The core may also be referred to herein as the core region.The cladding may also be referred to herein as the clad, clad region, orcladding region. The core has a higher refractive index than thecladding. The light diffusing element has a length with a lengthdimension and a cross-section with a cross-sectional dimension. Thelength dimension is the dimension in the axial direction of the elementand the cross-sectional dimension is a direction transverse to thedirection of light propagation. In the instance in which thelight-diffusing element has a rod configuration, for example, the lengthdimension of the light-diffusing element is the axial dimension, thecross-section may be circular, and the cross-sectional dimension may bethe diameter. It is to be understood, however, that the cross-section ofthe light-diffusing element may be arbitrarily shaped and may includeround or flat sides. Shapes of the cross-section may include circle,oval, square, rectangle, and polygon as well as shapes that include acombination of round and flat sides. As used herein, cross-sectionaldimension refers to the longest straight-line distance that connects twopoints of the outline (e.g. circumference, perimeter) of thecross-section. By way of example: for circular cross-sections, thecross-sectional dimension is the diameter; for ellipticalcross-sections, the cross-sectional dimension is the length of the majoraxis; and for square or rectangular cross-sections, the cross-sectionaldimension is the distance between opposite corners. It is furtherunderstood that the shape and/or dimensions of the cross-section may beconstant or variable along the length dimension of the light-diffusingelement. A light-diffusing element having a circular cross-section, forexample, may be tapered, where the diameter of the circularcross-section varies along the length of the light-diffusing element.

FIG. 1 depicts the cross-section of a light-diffusing element having acircular cross section. Light-diffusing element 10 includes core 12 andcladding 14. Light-diffusing element 10 may optionally include aprotective coating (not shown) surrounding cladding 14. Thecross-sectional dimension of the core may be at least 65 μm, or at least80 μm, or at least 100 um, or at least 150 μm, or at least 200 μm, or atleast 250 μm, or at least 300 μm, or between 65 μm and 500 μm, orbetween 100 μm and 400 μm, or between 200 μm and 350 μm. The thicknessof cladding 14 may be at least 10 μm, or at least 15 μm, at least 20 μm,at least 25 μm, or between 10 μm and 80 μm, or between 10 μm and 40 μm,or between 15 μm and 35 μm, or between 20 μm and 30 μm. When present, anoptional protective coating surrounding cladding 14 may have a thicknessof at least 20 μm, or at least 40 μm, at least 60 μm, at least 80 μm, orbetween 20 μm and 120 μm, or between 30 μm and 100 μm, or between 40 μmand 80 μm.

In one embodiment, the combined cross-sectional dimension of the coreand cladding of the light-diffusing element are significantly greaterthan the typical cross-sectional dimension of ˜125 μm for the combinedcore and cladding regions of a conventional transmission optical fiber.The light capture efficiency of a waveguide is proportional to itsetendue, which is defined as G=S(NA)², where S is the cross-sectionalarea and NA is the numerical aperture of the waveguide. A largercross-sectional dimension of the light-diffusing element leads to anincrease in etendue relative to a conventional transmission opticalfiber and this increased numerical aperture improves the efficiency ofcoupling to LED and laser diode light sources. A typical LED source, forexample, has a cross-sectional area of 1 mm² or higher and a numericalaperture (NA) of ˜0.9, while the typical transmission optical fiber hasa cross-sectional area of ˜0.2 mm² and a numerical aperture of ˜0.5 orless.

The length of light-diffusing element 10 may be at least 1 cm, or atleast 5 cm, or at least 20 cm, or at least 50 cm, or at least 100 cm, orbetween 1 cm and 1000 cm, or between 1 cm and 100 cm, or between 1 cmand 50 cm, or between 1 cm and 20 cm, or between 5 cm and 100 cm, orbetween 5 cm and 50 cm, or between 5 cm and 20 cm.

The core is glass, such as silica glass or modified silica glass. Thecladding may be glass or a polymer. Cladding glasses include silicaglass or modified silica glass. Cladding polymers include acrylatepolymers.

In one embodiment, the cladding polymer is the cured product of acladding composition that includes a curable crosslinker, a curablediluent, and a polymerization initiator. The cladding composition mayinclude one or more curable crosslinkers, one or more curable diluents,and/or one or more polymerization initiators. In one embodiment, thecurable crosslinker is essentially free of urethane and urea functionalgroups.

As used herein, the term “curable” is intended to mean that thecomponent, when exposed to a suitable source of curing energy, includesone or more curable functional groups capable of forming covalent bondsthat participate in linking the component to itself or to othercomponents to form the polymeric cladding material (i.e., the curedproduct). The curing process may be induced by radiation or by thermalenergy. A radiation-curable component is a component that can be inducedto undergo a curing reaction when exposed to radiation of a suitablewavelength at a suitable intensity for a sufficient period of time. Theradiation curing reaction may occur in the presence of a photoinitiator.A radiation-curable component may also optionally be thermally curable.Similarly, a thermally-curable component is a component that can beinduced to undergo a curing reaction when exposed to thermal energy ofsufficient intensity for a sufficient period of time. A thermallycurable component may also optionally be radiation curable.

A curable component may include one or more curable functional groups. Acurable component with only one curable functional group may be referredto herein as a monofunctional curable component. A curable componenthaving two or more curable functional groups may be referred to hereinas a multifunctional curable component or a polyfunctional curablecomponent. Multifunctional curable components include two or morefunctional groups capable of forming covalent bonds during the curingprocess and can introduce crosslinks into the polymeric network formedduring the curing process. Multifunctional curable components may alsobe referred to herein as “crosslinkers” or “curable crosslinkers”.Examples of functional groups that participate in covalent bondformation during the curing process are identified hereinafter.

In the description of the cladding composition that follows, variouscomponents of cladding compositions used to form cladding polymers willbe discussed and the amounts of particular components in the claddingcomposition will be specified in terms of weight percent (wt %) or partsper hundred (pph). The components of the cladding composition includebase components and additives. The concentration of base components willbe expressed in terms of wt % and the concentration of additives will beexpressed in terms of pph.

As used herein, the weight percent of a particular base component refersto the amount of the component present in the cladding composition on abasis that excludes additives. The additive-free cladding compositionincludes only base components and may be referred to herein as a basecomposition or base cladding composition. Any crosslinker component(s),diluent component(s), and polymerization initiator(s) present in acladding composition are regarded individually as base components andcollectively as a base composition. The base composition minimallyincludes a radiation-curable component and a polymerization initiator.The radiation-curable component may be a radiation-curable crosslinkeror a radiation-curable diluent. The base composition may, however,include one or more radiation-curable crosslinker components, one ormore radiation-curable diluent components, one or morenon-radiation-curable components, and one or more polymerizationinitiators. The collective amount of base components in a claddingcomposition is regarded herein as equaling 100 weight percent.

Additives are optional and may include one or more of an adhesionpromoter, an antioxidant, a catalyst, a carrier or surfactant, atackifier, a stabilizer, and an optical brightener. Representativeadditives are described in more detail hereinbelow. The amount ofadditives introduced into the cladding composition is expressed hereinin parts per hundred (pph) relative to the base composition. Forexample, if 1 g of a particular additive is added to 100 g of basecomposition, the concentration of additive will be expressed herein as 1pph.

In one embodiment, the curable crosslinker is a radiation curablecomponent of the cladding composition, and as such it includes one ormore functional groups capable of participating in the covalent bondingor crosslinking of the crosslinker into the polymeric cladding material.In one embodiment, the curable crosslinker includes two or moreradiation-curable functional groups. Exemplary functional groups capableof participating in the crosslinking include α,β-unsaturated ester,amide, imide or vinyl ether groups.

In one embodiment, the curable crosslinker is essentially free ofurethane or urea groups. The curable crosslinker may also be essentiallyfree of thiourethane or thiourea groups. By “essentially free” it ispreferable that less than 1 weight percent of the curable crosslinkercomponent includes (thio)urethane or (thio)urea groups. In preferredembodiments, less than 0.5 weight percent of the total curablecrosslinker component includes (thio)urethane or (thio)urea groups. Inmost preferred embodiments, the curable crosslinker component isentirely free of both (thio)urethane and (thio)urea groups.

When identifying certain groups, such as urethane and thiourethanegroups, or urea and thiourea groups, or isocyanate or thioisocyanategroups, these groups may be generically identified herein as(thio)urethane, (thio)urea, or (thio)isocyanate or di(thio)isocyanate toindicate that the sulfur atom(s) may or may not be present in the group.Such groups may be referred to herein as (thio)groups and componentscontaining (thio)groups may be referred to herein as (thio)components.The present embodiments extend to cladding compositions that include(thio)components with sulfur atom(s) or without sulfur atom(s) in the(thio)functional group as well as compositions that include some(thio)components with sulfur atom(s) and some (thio)components withoutsulfur atom(s).

In certain embodiments, the curable crosslinker component includes oneor more polyols that contain two or more α,β-unsaturated ester, amide,imide, or vinyl ether groups, or combinations thereof. Exemplary classesof these polyol crosslinkers include, without limitation, polyolacrylates, polyol methacrylates, polyol maleates, polyol fumarates,polyol acrylamides, polyol maleimides or polyol vinyl ethers comprisingmore than one acrylate, methacrylate, maleate, fumarate, acrylamide,maleimide or vinyl ether group. The polyol moiety of the curablecrosslinker can be a polyether polyol, a polyester polyol, apolycarbonate polyol, or a hydrocarbon polyol.

The curable crosslinker component preferably has a molecular weight ofbetween about 150 g/mol and about 15000 g/mol, in some embodiments morepreferably between about 200 g/mol and about 9000 g/mol, in someembodiments preferably between about 1000 g/mol and about 5000 g/mol, inother embodiments preferably between about 200 g/mol and about 1000g/mol. The curable crosslinker may further have a molecular weight inthe range from 100 g/mol to 3000 g/mol, or in the range from 150 g/molto 2500 g/mol, or in the range from 200 g/mol to 2000 g/mol, or in therange from 500 g/mol to 1500 g/mol.

The curable crosslinker component is present in the cladding compositionin an amount of about 1 to about 20 percent by weight, or in an amountof about 2 to about 15 percent by weight, or in an amount of about 3 toabout 10 percent by weight.

The curable diluent is a generally lower molecular weight (i.e., about120 to 600 g/mol) liquid monomer that is added to the formulation tocontrol the viscosity to provide the fluidity needed to apply thecoating composition with conventional liquid coating equipment. Thecurable diluent contains at least one functional group that allows thediluent, upon activation during curing, to link to the polymer formedduring the curing process from the curable crosslinker and other curablecomponents. Functional groups that may be present in the curable diluentinclude, without limitation, acrylate, methacrylate, maleate, fumarate,maleimide, vinyl ether, and acrylamide groups.

Monofunctional diluents will contain only a single reactive (curable)functional group, whereas polyfunctional diluents will contain two ormore reactive (curable) functional groups. Whereas the former can linkto the cladding polymer network during curing, the latter can formcrosslinks within the cladding polymer network.

When it is desirable to utilize moisture-resistant components, thediluent component will be selected on the basis of its compatibilitywith the selected moisture-resistant crosslinker(s) or component(s). Notall such liquid monomers may be successfully blended and copolymerizedwith the moisture-resistant crosslinker(s) or component(s) because suchcrosslinker(s) or component(s) are highly non-polar. For satisfactorycladding composition compatibility and moisture resistance, it isdesirable to use a liquid acrylate monomer component comprising apredominantly saturated aliphatic mono- or di-acrylate monomer or alkoxyacrylate monomers.

Suitable polyfunctional ethylenically unsaturated monomer diluentsinclude, without limitation, methylolpropane polyacrylates with andwithout alkoxylation such as ethoxylated trimethylolpropane triacrylatewith the degree of ethoxylation being 3 or greater, preferably rangingfrom 3 to about 30 (e.g. Photomer 4149 available from IGM Resins, andSR499 available from Sartomer Company, Inc.), propoxylatedtrimethylolpropane triacrylate with the degree of propoxylation being 3or greater, preferably ranging from 3 to 30 (e.g. Photomer 4072available from IGM Resins; and SR492 and SR501 available from SartomerCompany, Inc.), and ditrimethylolpropane tetraacrylate (e.g. Photomer4355 available from IGM Resins); alkoxylated glyceryl triacrylates suchas propoxylated glyceryl triacrylate with the degree of propoxylationbeing 3 or greater (e.g. Photomer 4096 available from IGM Resins; andSR9020 available from Sartomer Company, Inc.); erythritol polyacrylateswith and without alkoxylation, such as pentaerythritol tetraacrylate(e.g. SR295 available from Sartomer Company, Inc.), ethoxylatedpentaerythritol tetraacrylate (e.g. SR494 available from SartomerCompany, Inc.), and dipentaerythritol pentaacrylate (e.g. Photomer 4399available from IGM Resins; and SR399 available from Sartomer Company,Inc.); isocyanurate polyacrylates formed by reacting an appropriatefunctional isocyanurate with an acrylic acid or acryloyl chloride, suchas tris-(2-hydroxyethyl)isocyanurate triacrylate (e.g. SR368 availablefrom Sartomer Company, Inc.) and tris-(2-hydroxyethyl)isocyanuratediacrylate; alcohol polyacrylates with and without alkoxylation such astricyclodecane dimethanol diacrylate (e.g. CD406 available from SartomerCompany, Inc.), alkoxylated hexanediol diacrylate (e.g. CD564 availablefrom Sartomer Company, Inc.), tripropylene glycol diacrylate (e.g. SR306available from Sartomer Company, Inc.) and ethoxylated polyethyleneglycol diacrylate with a degree of ethoxylation being 2 or greater,preferably ranging from about 2 to 30; epoxy acrylates formed by addingacrylate to bisphenol A diglycidylether and the like (e.g. Photomer 3016available from IGM Resins); and single and multi-ring cyclic aromatic ornon-aromatic polyacrylates such as dicyclopentadiene diacrylate.

It may also be desirable to use certain amounts of monofunctionalethylenically unsaturated monomer diluents, which can be introduced toinfluence the degree to which the cured product absorbs water, adheresto other cladding composition materials, or behaves under stress.Exemplary monofunctional ethylenically unsaturated monomer diluentsinclude, without limitation, hydroxyalkyl acrylates such as2-hydroxyethyl-acrylate, 2-hydroxypropyl-acrylate, and2-hydroxybutyl-acrylate; long- and short-chain alkyl acrylates such asmethyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate,butyl acrylate, amyl acrylate, isobutyl acrylate, t-butyl acrylate,pentyl acrylate, isoamyl acrylate, hexyl acrylate, heptyl acrylate,octyl acrylate, isooctyl acrylate (e.g. SR440 available from SartomerCompany, Inc. and Ageflex FA8 available from CPS Chemical Co.),2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, isodecyl acrylate(e.g. SR395 available from Sartomer Company, Inc.; and Ageflex FA10available from CPS Chemical Co.), undecyl acrylate, dodecyl acrylate,tridecyl acrylate (e.g. SR489 available from Sartomer Company, Inc.),lauryl acrylate (e.g. SR335 available from Sartomer Company, Inc.,Ageflex FA12 available from CPS Chemical Co. (Old Bridge, N.J.), andPhotomer 4812 available from IGM Resins), octadecyl acrylate, andstearyl acrylate (e.g. SR257 available from Sartomer Company, Inc.);aminoalkyl acrylates such as dimethylaminoethyl acrylate,diethylaminoethyl acrylate, and 7-amino-3,7-dimethyloctyl acrylate;alkoxyalkyl acrylates such as butoxylethyl acrylate, phenoxyethylacrylate (e.g. SR339 available from Sartomer Company, Inc., Ageflex PEAavailable from CPS Chemical Co., and Photomer 4035 available from IGMResins), phenoxyglycidyl acrylate (e.g. CN131 available from SartomerCompany, Inc.), lauryloxyglycidyl acrylate (e.g. CN130 available fromSartomer Company, Inc.), and ethoxyethoxyethyl acrylate (e.g. SR256available from Sartomer Company, Inc.); single and multi-ring cyclicaromatic or non-aromatic acrylates such as cyclohexyl acrylate, benzylacrylate, dicyclopentadiene acrylate, dicyclopentanyl acrylate,tricyclodecanyl acrylate, bornyl acrylate, isobornyl acrylate (e.g.SR423 and SR506 available from Sartomer Company, Inc., and Ageflex IBOAavailable from CPS Chemical Co.), tetrahydrofurfuryl acrylate (e.g.SR285 available from Sartomer Company, Inc.), caprolactone acrylate(e.g. SR495 available from Sartomer Company, Inc.; and Tone M100available from Union Carbide Company, Danbury, Conn.), andacryloylmorpholine; alcohol-based acrylates such as polyethylene glycolmonoacrylate, polypropylene glycol monoacrylate, methoxyethylene glycolacrylate, methoxypolypropylene glycol acrylate, methoxypolyethyleneglycol acrylate, ethoxydiethylene glycol acrylate, and variousalkoxylated alkylphenol acrylates such as ethoxylated(4) nonylphenolacrylate (e.g. Photomer 4003 available from IGM Resins; and SR504available from Sartomer Company, Inc.) and propoxylatednonylphenolacrylate (e.g. Photomer 4960 available from IGM Resins); acrylamidessuch as diacetone acrylamide, isobutoxymethyl acrylamide,N,N′-dimethyl-aminopropyl acrylamide, N,N-dimethyl acrylamide,N,N-diethyl acrylamide, and t-octyl acrylamide; vinylic compounds suchas N-vinylpyrrolidone and N-vinylcaprolactam (both available fromInternational Specialty Products, Wayne, N.J.); and acid esters such asmaleic acid ester and fumaric acid ester.

The curable monomer diluent can include a single diluent component, orcombinations of two or more monomer diluent components. The curablemonomer diluent(s) is (are collectively) typically present in thecladding composition in amounts of about 10 to about 60 percent byweight, more preferably between about 20 to about 50 percent by weight,and most preferably between about 25 to about 45 percent by weight.

The cladding composition includes a polymerization initiator. Thepolymerization initiator is a reagent that is suitable to causepolymerization (i.e., curing) of the cladding composition after itsapplication to the core of the light-diffusing element. Polymerizationinitiators suitable for use in the cladding compositions include thermalinitiators, chemical initiators, electron beam initiators, andphotoinitiators. Photoinitiators are the preferred polymerizationinitiators. For most acrylate-based cladding polymer formulations,conventional photoinitiators, such as the known ketonic photoinitiatorsand/or phosphine oxide photoinitiators, are preferred. When used in thepresent cladding compositions, the photoinitiator is present in anamount sufficient to provide rapid ultraviolet curing. Generally, thisincludes between about 0.5 to about 10.0 percent by weight, morepreferably between about 1.5 to about 7.5 percent by weight.

The photoinitiator, when used in a small but effective amount to promoteradiation cure, should provide reasonable cure speed without causingpremature gelation of the coating composition. A desirable cure speed isany speed sufficient to cause substantial curing of the coatingmaterials.

Suitable photoinitiators include, without limitation,1-hydroxycyclohexylphenyl ketone (e.g. Irgacure 184 available fromBASF), (2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide(e.g. commercial blends Irgacure 1800, 1850, and 1700 available fromBASF), 2,2-dimethoxyl-2-phenyl acetophenone (e.g. Irgacure 651,available from BASF), bis(2,4,6-trimethyl benzoyl)phenyl-phosphine oxide(e.g. Irgacure 819, available from BASF),(2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (e.g. Lucerin TPOavailable from BASF, Munich, Germany),ethoxy(2,4,6-trimethylbenzoyl)phenyl phosphine oxide (e.g. Lucerin TPO-Lfrom BASF), and combinations thereof.

The polymeric cladding compositions may also include one or moreadditives. Representative additives include an adhesion promoter, anantioxidant, a catalyst, a carrier or surfactant, a tackifier, astabilizer, and an optical brightener. Some additives (e.g., catalysts,reactive surfactants, and optical brighteners) may operate to controlthe polymerization process and may thereby affect the physicalproperties (e.g., modulus, glass transition temperature) of the curedproduct formed from the cladding composition. Other additives mayinfluence the integrity of the cured product of the cladding composition(e.g., protect against de-polymerization or oxidative degradation).

An adhesion promoter enhances the adhesion of the primary coating to theunderlying glass fiber. Any suitable adhesion promoter can be employed.Examples of a suitable adhesion promoter include, without limitation,organofunctional silanes, titanates, zirconates, and mixtures thereof.One preferred class are the poly(alkoxy)silanes. Suitable alternativeadhesion promoters include, without limitation,bis(trimethoxysilylethyl)benzene, 3-mercaptopropyltrimethoxysilane(3-MPTMS, available from United Chemical Technologies, Bristol, Pa.;also available from Gelest, Morrisville, Pa.),3-acryloxypropyltrimethoxysilane (available from Gelest), and3-methacryloxypropyltrimethoxysilane (available from Gelest), andbis(trimethoxysilylethyl)benzene (available from Gelest). Other suitableadhesion promoters are described in U.S. Pat. Nos. 4,921,880 and5,188,864 to Lee et al., each of which is hereby incorporated byreference. The adhesion promoter, if present, is used in an amountbetween about 0.1 to about 10 pph, more preferably about 0.25 to about 3pph.

Any suitable antioxidant can be employed. Preferred antioxidantsinclude, without limitation, bis hindered phenolic sulfide orthiodiethylene bis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate (e.g.Irganox 1035, available from BASF), 2,6-di-t-butyl-4-methylphenol (BHT).The antioxidant, if present, is used in an amount between about 0.1 pphto about 3 pph, more preferably about 0.25 pph to about 2 pph.

Suitable carriers, more specifically carriers which function as reactivesurfactants, include polyalkoxypolysiloxanes. Exemplary preferredcarriers are available from Goldschmidt Chemical Co. (Hopewell, Va.)under the tradename TEGORAD 2200 and TEGORAD 2700 (acrylated siloxane).These reactive surfactants may be present in a preferred amount betweenabout 0.01 pph to about 5 pph, more preferably about 0.25 pph to about 3pph. Other classes of suitable carriers are polyols and non-reactivesurfactants. Examples of suitable polyols and non-reactive surfactantsinclude, without limitation, the polyol Acclaim 3201 (poly(ethyleneoxide-co-propylene oxide)) available from Bayer (Newtown Square, Pa.),and the non-reactive surfactant Tegoglide 435 (polyalkoxy-polysiloxane)available from Goldschmidt Chemical Co. The polyol or non-reactivesurfactants may be present in a preferred amount between about 0.01 pphto about 10 pph, more preferably about 0.05 pph to about 5 pph, mostpreferably about 0.1 pph to about 2.5 pph.

Suitable carriers may also be ambiphilic molecules. An ambiphilicmolecule is a molecule that has both hydrophilic and hydrophobicsegments. The hydrophobic segment may alternatively be described as alipophilic (fat/oil loving) segment. A tackifier is an example of onesuch ambiphilic molecule. A tackifier is a molecule that can modify thetime-sensitive rheological property of a polymer product. In general atackifier additive will make a polymer product act stiffer at higherstrain rates or shear rates and will make the polymer product softer atlow strain rates or shear rates. A tackifier is an additive that iscommonly used in the adhesives industry, and is known to enhance theability of a coating to create a bond with an object that the coating isapplied upon. One preferred tackifier is Uni-tac® R-40 (hereinafter“R-40”) available from International Paper Co., Purchase, N.Y. R-40 is atall oil rosin, which contains a polyether segment, and is from thechemical family of abietic esters. A suitable alternative tackifier isthe Escorez® series of hydrocarbon tackifiers available from Exxon. Foradditional information regarding Escorez® tackifiers, see U.S. Pat. No.5,242,963 to Mao, which is hereby incorporated by reference in itsentirety. The aforementioned carriers may also be used in combination.Preferably, the tackifier is present in the composition in an amountbetween about 0.01 pph to about 10 pph, more preferably in the amountbetween about 0.05 pph to about 5 pph.

Any suitable stabilizer can be employed. One preferred stabilizer is atetrafunctional thiol, e.g., pentaerythritoltetrakis(3-mercaptopropionate) from Sigma-Aldrich (St. Louis, Mo.). Thestabilizer, if present, is used in an amount between about 0.01 pph toabout 1 pph, more preferably about 0.01 pph to about 0.2 pph.

Any suitable optical brightener can be employed. Exemplary opticalbrighteners include, without limitation, Uvitex OB, a2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (BASF); BlankophorKLA, available from Bayer; bisbenzoxazole compounds; phenylcoumarincompounds; and bis(styryl)biphenyl compounds. The optical brightener isdesirably present in the composition at a concentration of about 0.003pph to about 0.5 pph, more preferably about 0.005 pph to about 0.3 pph.

To increase the numerical aperture of the light-diffusing element andimprove coupling efficiency to high numerical aperture light sources, itis desirable to maximize the refractive index contrast between the coreand cladding. One strategy for increasing the numerical aperture of thelight-diffusing element is to reduce the refractive index of the polymercladding. The refractive index of the polymer cladding can be reduced byincorporating fluorinated or partially fluorinated analogs of thecrosslinker(s) and/or monomer diluent(s) used in the claddingcomposition. Fluorine-substituted variants of the crosslinkers andmonomer diluents described hereinabove can be prepared using methodsknown in the art.

FIGS. 2-4 depict variations 10 a-10 c, respectively, of light diffusingelement 10 of FIG. 1 that include scattering centers in the core and/orcladding. In one embodiment, the core includes scattering centers andthe cladding lacks scattering centers. In another embodiment, the corelacks scattering centers and the cladding includes scattering centers.In still another embodiment, the core includes scattering centers andthe cladding includes scattering centers. When indicating herein that aregion (core or cladding) lacks scattering centers, it is intended tomean that no deliberate introduction of scattering centers into theregion has occurred. It will be appreciated, however, that imperfectionsor fluctuations in composition, density etc. may induce scattering.

FIG. 2 depicts light-diffusing element 10 a having core 12 a andcladding 14 a, where core 12 a includes scattering centers and cladding14 a lacks scattering centers. Enlargement 30 a of region 20 a of core12 a shows scattering centers 40 a. FIG. 3 depicts light-diffusingelement 10 b having core 12 b and cladding 14 b, where core 12 b lacksscattering centers and cladding 14 b includes scattering centers.Enlargement 30 b of region 20 b of core 12 b shows scattering centers 40b. FIG. 4 depicts light-diffusing element 10 c having core 12 c andcladding 14 c, where core 12 c includes scattering centers and cladding14 c includes scattering centers. Enlargement 30 c of region 20 c ofcore 12 c shows scattering centers 40 c and enlargement 30 d of region20 d of core 12 d shows scattering centers 40 d. For purposes ofillustration, scattering centers 40 a-40 d are shown as having uniformcross-sectional size and shape. In practice, a distribution of sizes(cross-sectional and/or length dimensions) and shapes for the scatteringcenters will be present. The size and shape distribution as well asnumber of scattering centers may vary within a cross-section, in thecore relative to the cladding, and/or along the length of thelight-diffusing element.

Scattering centers for the core and cladding include dopants,nanoparticles, and internal voids. Preferred scattering centers forglass regions of the light-diffusing element are dopants and internalvoids. Preferred scattering centers for polymer regions arenanoparticles and internal voids.

In one embodiment, the scattering centers are dopants. Dopants areelements that are incorporated into a base glass composition to modifythe refractive index. Dopants include updopants and downdopants. Anupdopant is a dopant that raises the refractive index of the base glasscomposition and a downdopant is a dopant that lowers the refractiveindex of the base glass composition. In one embodiment, the base glasscomposition (of the core and/or cladding) is silica glass. Up-dopantsfor silica glass include Ge, Al, P, Ti, Cl, and Br. Downdopants forsilica glass include F and B. The incorporation of dopants in a glasscore or glass cladding enhances scattering through the Rayleighscattering and/or small angle scattering mechanisms.

In another embodiment, the scattering centers are voids. Voids areinternal gas-filled regions within the core or cladding. Gases that fillthe internal void include SO₂, noble gases, CO₂, N₂, O₂, air, ormixtures thereof. The internal voids have a lower refractive index thanthe surrounding solid core or solid cladding material. When present,internal voids contribute to a reduction in the average refractive indexof the core or cladding and provide centers that scatter light. Theinternal voids may be distributed throughout the cross-section of thecore and/or cladding or localized within one or more discrete regionsthereof. The internal voids may be configured in a random ornon-periodic arrangement and may have a uniform or non-uniformdistribution of size or number.

In glass, the internal voids may have a cross-section with a dimensionof at least 50 nm, or at least 100 nm, or at least 500 nm, or between 50nm and 20 μm, or between 100 nm and 10 μm or between 500 nm and 10 μm,or between 500 nm and 5 μm. In cladding polymer, the internal voids mayhave a cross-section with a dimension or at least 25 nm, or at least 100nm, or at least 250 nm, or at least 500 nm, or at least 1000 nm, orbetween 25 nm and 40 μm, or between 100 nm and 40 μm, or between 250 nmand 40 μm, between 500 nm and 20 μm, or between 1000 nm and 10 μm. Inglass, the internal voids may have a length in the range from a fewmicrons to a several meters; for example between 1 μm and 50 μm, orbetween 10 μm and 30 μm, or between 100 μm and 20 μm, or between 1 μmand 1 μm, or between 1 μm and 100 cm, or between 1 μm and 10 cm, orbetween 10 μm and 10 μm or between 10 μm and 1 μm, or between m and 100cm, or between 10 μm and 10 cm, or between 100 μm and 1 μm. In claddingpolymer, the internal voids may have a length between 25 nm and 40 μm,or between 100 nm and 40 μm, or between 250 nm and 40 μm, or between 500nm and 20 μm, or between 500 nm and 40 μm, or between 500 nm and 20 μm,or between 500 nm and 10 μm, or between 500 nm and 5 μm. The internalvoids within the core or cladding may include a distribution ofcross-sectional dimensions and lengths.

In the core, the internal voids may occupy a fill fraction of between0.5% and 20% of the core, or between 1% and 15% of the core, or between2% and 10% of the core. In the cladding (glass or polymer), the internalvoids may occupy a fill fraction of between 0.5% and 30% of thecladding, or between 1% and 15% of the cladding, or between 2% and 10%of the cladding. As used herein, fill fraction refers to the fraction ofthe cross-sectional area occupied by the internal voids. In oneembodiment, the fill fraction is constant along the length of thelight-diffusing element. In another embodiment, the fill fraction variesalong the length of the light-diffusing element. To a goodapproximation, the fill fraction corresponds to the volume fraction ofinternal voids. The volume fraction of voids within the core may be atleast 0.5%, or at least 1.0%, or at least 2.0%, or at least 5.0%, orbetween 0.5% and 20%, or between 1% and 15%, or between 2% and 10%. Thevolume fraction of voids within the cladding (glass or polymer) may beat least 0.5%, or at least 1.0%, or at least 2.0%, or at least 5.0%, orbetween 0.5% and 30%, or between 1.0% and 15%, or between 2.0% and 10%,or between 2.0% and 30%, or between 3.0% and 20%.

The cross-sectional distribution of internal voids may vary at differentpositions in the core and/or cladding along the length of thelight-diffusing element. As noted, the length and cross-sectionalattributes (e.g. shape and size or density) of the internal voids mayvary. The variations may also occur in the axial or length direction ofthe light-diffusing element. Since the length of the internal voids maynot extend the full length of the light-diffusing element, particularinternal voids may be present in some cross-sections and absent in othercross-sections.

In a further embodiment, the scattering centers are nanoparticles.Nanoparticles are particulate matter having dimensions in the nanoscaleregime. The cross-sectional dimension of the nanoparticles may be atleast 25 nm, or at least 50 nm, or at least 100 nm, or at least 150 nm,or at least 200 nm, or between 25 nm and 500 nm, or between 50 nm and400 nm, or between 50 nm and 300 nm, or between 50 nm and 200 nm, orbetween 100 nm and 400 nm, or between 100 nm and 300 nm. In oneembodiment, the nanoparticles are approximately spherical and thecross-sectional dimension is the diameter of the sphere. Theconcentration of nanoparticles may be between 1% and 30% by volume, orbetween 2% and 25% by volume, or between 5% and 20% by volume.

In one embodiment, the nanoparticles are oxide nanoparticles.Representative compositions for oxide nanoparticles include TiO₂, ZrO₂,other transition metal oxides, rare earth oxides, mixed metal oxides(e.g. garnets such as Y₃Al₅O₁₂), SiO₂, and Al₂O₃. Fluoride orchalcogenide nanoparticles may also be used. In one embodiment, thenanoparticles are luminescent (e.g. fluorescent or phosphorescent).Luminescent nanoparticles include oxides that include a light-emittingmetal center. Light-emitting metal centers include Cr³⁺, Ce³⁺, Nd³⁺,Tb³⁺, Eu³, and Pr³⁺. Light-emitting metal centers may be incorporated asdopants in otherwise non-luminescent inorganic host lattices (e.g. oxidelattices such as Y₃Al₅O₁₂ or Al₂O₃). Luminescent nanoparticles alsoinclude quantum dots or light-emitting semiconductor materials such asCdS, CdSe, ZnTe, ZnS, or other direct bandgap II-VI or III-Vsemiconductor materials. Use of light-emitting scattering centerspermits control of the color of light emanating from the light-diffusingelement. The light-emitting scattering centers in the cladding absorb atleast a portion of the light scattered from the core and reemit thelight at a different wavelength to modify the color of the light.Light-emitting nanoparticles that emit at multiple wavelengths may beincorporated to provide greater control over color to achieve any color,or combinations of colors, in the spectrum (including white light).Inclusion of light-emitting scattering centers avoids the need to applya separate phosphor coating layer over the light-diffusing element.

In one embodiment, the light-diffusing element includes an undoped glasscore and a glass cladding with internal voids. In another embodiment,the light-diffusing element includes a doped glass core and a glasscladding with internal voids. In still another embodiment, thelight-diffusing element includes an undoped glass core and a polymercladding with internal voids. In yet another embodiment, thelight-diffusing element includes a doped glass core and a polymercladding with internal voids. In a further embodiment, thelight-diffusing element includes an undoped glass core and a polymercladding with nanoparticles. In an additional embodiment, thelight-diffusing element includes a doped glass core and a polymercladding with nanoparticles.

In one embodiment, the light-diffusing element includes a glass corewith internal voids and a glass cladding with internal voids. In anotherembodiment, the light-diffusing element includes a glass core withinternal voids and a polymer cladding with internal voids. In stillanother embodiment, the light-diffusing element includes a glass corewith internal voids and a polymer cladding with nanoparticles.

The core and/or cladding may include more than one type of scatteringcenter. In one embodiment, the core is a glass that includes a dopantand internal voids. In another embodiment, the cladding is a glass thatincludes a dopant and internal voids. In still another embodiment, thecladding is a polymer that includes internal voids and nanoparticles.

The light-diffusing element may be configured to scatter light along allor some of its length by controlling the placement and concentration ofthe scattering centers in the core and/or cladding. Regions of thelight-diffusing element that include scattering centers may efficientlyscatter light to produce an illumination effect, while regions of thelight-diffusing element that lack scattering centers may not. Asdescribed more fully below, processing conditions may be used to controlwhether scattering centers form in a particular region of thelight-diffusing element and the spatial and dimensional characteristicsof internal voids that do form. The light-diffusing element may includecross-sections or extended lengths of solid glass without scatteringcenters that scatter little or no light along with cross-sections orextended lengths that include scattering centers. Regions orcross-sections with and without scattering centers may be interspersedor alternating along the length of the light-diffusing element.

The brightness of the light-diffusing element as an illumination sourcedepends on the intensity of scattered light that passes through theouter surface. The intensity of scattered light depends on thescattering loss of light propagating through the light-diffusingelement. As used herein, scattering loss refers to light directedoutside of the light-diffusing element by the combined scattering of thecore and cladding. A higher scattering loss leads to a greater intensityof scattered light per unit length of the light-diffusing element andincreases the brightness of the light-diffusing element. The scatteringloss of the light-diffusing element may be at least 0.1 dB/m, or atleast 0.5 dB/m, or at least 1 dB/m, or at least 2 dB/m, or at least 5dB/m, or at least 10 dB/m.

In some applications, it may be desirable to achieve uniformity inillumination intensity along the length of the light-diffusing elementsor selected regions thereof. The intensity of scattered light thatpasses through the outer surface of the element may have a maximumvalue. The variation in the intensity of the scattered light that passesthrough the outer surface of the element may vary by less than 50% ofthe maximum value along the length of the element or selected portionsthereof at the illumination wavelength, or less than 30% of the maximumvalue along the length of the element or selected portions thereof atthe illumination wavelength, or less than 20% of the maximum value alongthe length of the element or selected portions thereof at theillumination wavelength.

The scattering efficiency may vary along the length of thelight-diffusing element. It may be desirable to control the degree ofvariation of scattering of the illumination wavelength along the lengthof the light-diffusing element or selected portions thereof to achieve amore uniform illumination effect. The scattering efficiency may bedifferent near the source end of the element relative to near thedelivery end of the element. The scattering efficiency may increasealong the element with increasing distance from the source end of theelement. Higher scattering efficiency at positions more distant from thesource act to preserve illumination intensity by compensating for lossesin the intensity of source light with increasing distance from thesource. As light propagates away from the source in the light-diffusingelement, it scatters and its intensity progressively decreases. Tomaintain brightness, the light-diffusing element can be configured toprovide an increasing scattering efficiency as the intensity of sourcelight diminishes with increasing distance from the source. Scatteringefficiency can be controlled, for example, by varying the concentration,size and/or composition of scattering centers along the length of thelight-diffusing element.

The scattering efficiency along the length of the element may have amaximum value. The scattering efficiency at the illumination wavelengthalong the length of the light-diffusing element or selected portionsthereof may vary by less than 50% of the maximum value, or less than 30%of the maximum value, or less than 20% of the maximum value. The statedvariations in scattering efficiency, scattered light intensity, and/orthe stated scattering losses may be simultaneously realized in thelight-diffusing element.

Inclusion of scattering centers in the cladding obviates the need for aseparate outer surface ink layer on the light-diffusing element. As isknown in the art, ink layers are commonly applied to light-diffusingelements to control or modify the angular distribution of lightscattered from the light-diffusing element. The ink layer may beutilized to enhance the distribution and/or the nature of the scatteredlight. The ink layer may make the angular distribution of lightscattered from the light-diffusing element more uniform by compensatingfor directional bias (e.g. forward scattering vs. backward scattering)in the scattering of light from the core region to provide greaterangular uniformity in the scattering of light. Angle independence in thedistribution of light scattered from the light-diffusing elementpromotes a more uniform intensity distribution in the angular direction.The presence of scattering centers in the cladding inherently providesthe benefits of a surface ink layer.

Glass portions of the light-diffusing element may be made by forming asoot-containing optical fiber preform via chemical vapor deposition(CVD), outer vapor deposition (OVD), vapor axial deposition (VAD), flamehydrolysis, flame oxidation, or other techniques known in the art.Dopants can be provided in the glass composition used to make thepreform. The preform can include a core region and/or a cladding region,where doping may differ in each region.

To form internal voids, the soot preform may be consolidated in agaseous atmosphere that surrounds the preform. Consolidation in thepresence of the gaseous atmosphere causes a portion of the gaseousatmosphere to become trapped in the preform during consolidation,thereby resulting in the formation of internal voids in the consolidatedpreform. The voids may be non-periodically distributed in theconsolidated preform and each void may correspond to a region of atleast one trapped consolidated gas within the consolidated glasspreform. The consolidated preform with voids is then drawn to make alight-diffusing element in accordance with the present disclosure. Atleast some of the voids formed in the preform during consolidationremain in the drawn element. A light-diffusing fiber or multiplelight-diffusing fibers may be utilized in place of a fiber perform.

The conditions under which consolidation occurs may be manipulated tocontrol the size, shape, length, fill fraction, and spatial distributionof voids. Directional control of void characteristics (e.g. along theaxial vs. transverse directions) may also be achieved. The consolidationconditions may be effective to result in a significant amount of gasesbeing trapped in the consolidated glass blank, thereby causing theformation of non-periodically distributed voids in the consolidatedglass preform. The resultant preform is used to form a light-diffusingelement with voids therein. By utilizing relatively low permeabilitygases and/or relatively high sintering rates, holes can be trapped inthe consolidated glass during the consolidation process. During the sootconsolidation step, the soot goes through a densification process viaexposure to high heat to remove the open porosity (e.g. pores betweenthe soot which is not surrounded by densified glass) and leavingdensified glass. In the context of the present disclosure, the trappingof substantial amounts of the ambient gas present in the consolidationprocess precludes full densification of the glass and voids remain inthe glass after consolidation. Soot consolidation may be performed in asoot consolidation furnace. The sintering rate can be increased byincreasing the sintering temperature and/or increasing the downfeed rateof the soot preform through the sintering zone of the consolidationfurnace. Under certain sintering conditions, it is possible to obtainglasses in which the area fraction of the trapped gases is a significantfraction of the total area or volume of the preform.

A soot preform may be formed by depositing silica-containing soot ontoan outer surface of a rotating and translating mandrel or bait rod. Thisprocess is known as the OVD or outside vapor deposition process. Themandrel is preferably tapered and the soot is formed by providing aglass precursor in gaseous form to the flame of a burner to oxidize it.A fuel, such as methane (CH₄) and a supporting combustion gas, such asoxygen, is provided to the burner and ignited to form the flame. Glassformer compounds (e.g. SiCl₄, octamethylcyclotetrasiloxane) are oxidizedin the flame to form a generally cylindrically-shaped soot region on amandrel or substrate. A dopant compound may be included.

The soot preform may be consolidated in a consolidation furnace to forma consolidated blank. Prior to consolidation, the mandrel is removed toform a hollow, cylindrical soot blank preform. During the consolidationprocess, the soot preform is suspended, for example, inside a purequartz muffle tube of the consolidation furnace by a holding mechanism.Preferably, before the consolidation step, the preform is exposed to adrying atmosphere. For example, a suitable drying atmosphere may includeabout 95% to 99% helium and 1% to 5% chlorine gas at a temperature ofbetween about 950° C. and 1250° C. and a suitable drying time rangesfrom about 0.5 and 4.0 hours.

During the consolidation step, which preferably takes place after a sootdrying step, the furnace temperature is raised and the preform isconsolidated at a suitable temperature, for example between about 1390°C. and 1535° C. to form a consolidated preform.

Gradient sintering may be employed whereby the soot preform is drivendown through a hot zone of the furnace, which is maintained at atemperature of between about 1225° C. to 1550° C., or between about1390° C. and 1535° C. For example, the preform may be held in anisothermal zone which is maintained at a desired drying temperature(950-1250° C.), after which the soot preform is driven through a zonewhich is maintained at a desired consolidation temperature (e.g. between1225° C. and 1550° C., or between 1390° C. and 1535° C.) at a rate ofspeed which is sufficient to result in the preform temperatureincreasing by greater than 1° C./min. Upper zones of the furnace can bemaintained at lower temperatures which facilitate a drying and impurityremoval step. The lower zone can be maintained at the highertemperatures desired for consolidation. The soot containing preform maydownfed through a consolidation hot zone at a first downfeed rate,followed by downfeeding of the preform through a second hot zone at asecond downfeed rate which is less than the first downfeed rate. Such aconsolidation technique results in the outside portion of the sootpreform sintering before the rest of the preform sinters, therebyfacilitating trapping of gases which will in turn facilitate formationof and retaining of voids in the resultant consolidated glass.

For example, the preform can be exposed to such suitable consolidationtemperatures (e.g. greater than about 1390° C.) at a first speed whichis sufficient to result in the preform temperature increasing by morethan 15° C./min, more preferably greater than 17° C./min, followed by atleast a second downfeed rate/consolidation temperature combination whichis sufficient to result in the preform heating by at least about 12°C./min, more preferably greater than 14° C./min. Preferably, the firstconsolidation rate results in the outside of the preform increasing intemperature at a rate which is greater than 2° C., or greater than 10°C., or greater than about 20° C., and most preferably greater than 50°C./min higher than the heating rate of the second consolidation rate. Ifdesired, a third consolidation step or even further consolidation stepscan be employed which heats at a slower rate (e.g. less than 10°C./min). Alternatively, the soot preform can be sintered at even fasterrates in order to create more voids by driving the soot preform througha furnace hot zone where the temperature is greater than 1550° C., orgreater than 1700° C., even more preferably greater than 1900° C.Alternatively, the soot preform can be sintered at even faster ratesexternal to the furnace by using an open flame or plasma torch incontact with the soot.

Voids may be formed by exposing the preform to a gas during sinteringand/or consolidation. The gas used to form internal voids in the preformmay be referred to herein as a void-producing gas. Preferredvoid-producing gases include one or more of N₂, Ar, Kr, CO₂, O₂, air,SO₂, Cl₂, CF₄, or mixtures thereof. Void-producing gases may be useddirectly or in the presence of a diluent gas during sintering orconsolidation. Each of the void-producing gases exhibits a relativelylow permeability in silica glass at or below the consolidationtemperature that is suitable for forming internal voids in accordancewith the methods present disclosure. Preferably these void-producinggases are employed either alone or in combination in an amount between5% and 100% by volume, or between about 20% and 100% by volume, orbetween about 40% and 100% by volume. The remainder of the sintering orconsolidation gas atmosphere is made up of a suitable diluent or carriergas such as, for example, helium, hydrogen, deuterium, or mixturesthereof. Generally speaking, the greater the percentage ofvoid-producing gases employed in the sintering gas, the larger and moreabundant the voids will be in the resultant consolidated glass. As aresult, control of the size, shape, number density, and spatialdistribution of internal voids can be achieved through variations in thetype of void-producing gas, the pressure of the void-producing gas inthe processing environment, and/or the ratio of void-producing gas todiluent gas.

When it is desired to deposit additional soot via OVD to the resultantglass perform or cane subsequent to the void-producing consolidationprocess, a sintering gas that includes less than 10% O₂, or less than 5%O₂, or no O₂ may be employed to avoid loss of seeds upon exposure tohydrogen formed in the OVD process. The void-producing gas may be acombination of N₂ and Ar, where the combination of N₂ and Ar is employedin the sintering atmosphere in an amount greater than 10% by volume, orgreater than 30% by volume, or greater than 50% by volume.

Using the sintering gases described herein, it is desirable to employ aconsolidation process which includes a downfeed of the preform at a rateand temperature which is sufficient to result in at least some of theconsolidation gases being intentionally trapped. This can occur, forexample, by heating of at least a portion of the soot preform greaterthan about 10° C./min, more preferably greater than about 12° C./min,even more preferably greater than about 14° C./min. The sinteringtemperatures employed in the present invention preferably are greaterthan 1100° C., or greater than 1300° C., or greater than 1400° C., orand greater than 1450° C.

The gaseous atmosphere employed during the consolidation process, thetemperature inside the consolidation furnace, and preform consolidationrate may be selected so that, during the soot consolidation process,gases are intentionally trapped within the preform, forming internalvoids in the consolidated glass. These gas containing internal voids arepreferably not entirely outgassed prior to and/or during the elementdrawing process, so that the internal voids remain in the glass afterthe glass has been drawn. A variety of process parameters can becontrolled to vary and control the size of the voids. For example,increasing the consolidation time or temperature can increase the voidsize, as the increased temperature causes the gases trapped within thevoids to expand. Similarly, the size and area percent of the voids canbe impacted by the draw conditions. For example, a longer hot zone in adraw furnace and/or a faster draw speed tends to increase the size aswell as the area percent of the voids. Selection of a gas that is morepermeable in glass at the consolidation temperature will result insmaller voids.

Sintering rate can also have a significant effect on void size and voiddensity. A faster sintering rate will result in the formation of moreand larger voids. However, use of sintering rates that are too slow willresult in no voids being formed, as the gas will have time to escapethrough the glass. Consequently, the downfeed rate of the preform and/orthe consolidation temperature employed are preferably high enough toresult in the heating of at least a portion of the preform at a rategreater than about 10° C./min, more preferably greater than about 12°C./min, even more preferably greater than about 14° C./min. Generallyspeaking, a preform having a lower soot density will result in formationof more voids. However, the density of the deposited soot in aparticular preform can be varied to position more voids (higher regionalvoid area percent) where desired. For example, a first high density sootregion can be deposited directly onto a consolidated glass (e.g. puresilica) core cane, followed by a second region of soot having a lowerdensity than that of the first. We have found that this causes a highervoid area percent to form near the core (i.e. in the high density sootregion). The silica containing soot preferably has a bulk density ofbetween about 0.10 g/cc and 1.7 g/cc, more preferably between about 0.30g/cc and 1.0 g/cc. This effect can also be used to form consolidatedvoid containing preforms which alternate between low or no voidcontaining regions and higher void containing regions; wherein theinitial soot density radial variation is greater than 3 percent over adistance of at least 100 μm. Additional information about fabricationand processing may be found in U.S. Pat. No. 7,450,806, the disclosureof which is hereby incorporated by reference herein.

A polymer cladding can be formed on a glass core (doped or undoped, withor without internal voids) by applying a cladding composition of thetype described hereinabove to the glass core during the process ofdrawing the core from a preform. The cladding composition is a liquidand can be sprayed, brushed or otherwise applied to the drawn glasscore. Reaction of the cladding composition provides a polymer claddingon the glass core. In one embodiment, the reaction is a photoreactionand polymerization (curing) of the cladding composition occurs uponexposing the applied coating composition to radiation of properwavelength. The proper wavelength is a wavelength capable of initiatingthe curing reaction and may be dictated by the choice of photoinitiatorused in the cladding composition.

Internal voids may be formed in the polymer cladding by performing thecuring in the presence of a void-producing gas. As the curing reactionproceeds, cladding composition becomes increasingly viscous and thevoid-producing gas gets trapped in the polymer to form internal voids.To retain voids, it is preferable that the void-producing gas beinsoluble or only weakly soluble in the polymer cladding. Thevoid-producing gas should also have a low diffusion coefficient in thepolymer cladding. Suitable void-producing gases for the polymer claddinginclude O₂, N₂, air, and Ar. The size, shape, and number density ofinternal voids in the polymer cladding can be controlled by varying thetype of void-producing gas, the time of exposure of the claddingcomposition during curing to the void-producing gas, and the pressure orconcentration of void-producing gas in the curing environment.

In one embodiment, the characteristics of internal voids formed in thepolymer cladding are varied by including a gas that is soluble in thecladding polymer along with a void-producing gas in the curingenvironment. The soluble gas dilutes the void-producing gas and amixture of the soluble gas and void-producing gas is initiallyincorporated as an internal void. As curing proceeds and/or time passes,the soluble gas exits the internal void and dissolves in the claddingpolymer, leading to a partial collapse (shrinkage) of the internal void.By controlling the relative proportions of soluble gas andvoid-producing gas, it is possible to systematically vary the size ofinternal voids. A high proportion of soluble gas in a mixture with thevoid-producing gas is expected to produce smaller internal voids onaverage and vice versa. In one embodiment, the soluble gas is CO₂.

Nanoparticles may be incorporated in the polymer cladding by addingnanoparticles (directly as a solid, in the form of a suspension, or in asolvent) to the cladding composition before applying the claddingcomposition to the drawn glass core. As the curing reaction proceeds andthe polymer cladding forms, the nanoparticles become incorporated in thepolymer cladding. In one embodiment, the nanoparticles are incorporatedas a dispersed phase in the polymer cladding.

The light-diffusing element may include a protective coating surroundingthe cladding. The protective coating is intended to protect thelight-diffusing element from mechanical damage. The protective coatingmay be formed from a curable protective coating composition thatincludes one or more monomers. The monomers may include ethylenicallyunsaturated compounds. The curable protective coating composition mayalso include one or more oligomers, one or more polymerizationinitiators, and one or more additives. In one embodiment, the protectivecoating is the polymerization product of a protective coatingcomposition that contains urethane acrylate monomers.

The monomer component of the curable protective coating composition mayinclude one or more monomers. The one or more monomers may be present inan amount of 50 wt % or greater, or in an amount from about 75 wt % toabout 99 wt %, or in an amount from about 80 wt % to about 99 wt % or inan amount from about 85 wt % to about 98 wt %.

The monomer component of the curable protective coating composition mayinclude ethylenically unsaturated compounds. The ethylenicallyunsaturated monomers may be monofunctional or polyfunctional. Thefunctional groups may be polymerizable groups and/or groups thatfacilitate or enable crosslinking. In combinations of two or moremonomers, the constituent monomers may be monofunctional,polyfunctional, or a combination of monofunctional and polyfunctionalcompounds. Suitable functional groups for ethylenically unsaturatedmonomers include, without limitation, (meth)acrylates, acrylamides,N-vinyl amides, styrenes, vinyl ethers, vinyl esters, acid esters, andcombinations thereof.

Exemplary monofunctional ethylenically unsaturated monomers include,without limitation, hydroxyalkyl acrylates such as2-hydroxyethyl-acrylate, 2-hydroxypropyl-acrylate, and2-hydroxybutyl-acrylate; long- and short-chain alkyl acrylates such asmethyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate,butyl acrylate, amyl acrylate, isobutyl acrylate, t-butyl acrylate,pentyl acrylate, isoamyl acrylate, hexyl acrylate, heptyl acrylate,octyl acrylate, isooctyl acrylate, 2-ethylhexyl acrylate, nonylacrylate, decyl acrylate, isodecyl acrylate, undecyl acrylate, dodecylacrylate, lauryl acrylate, octadecyl acrylate, and stearyl acrylate;aminoalkyl acrylates such as dimethylaminoethyl acrylate,diethylaminoethyl acrylate, and 7-amino-3,7-dimethyloctyl acrylate;alkoxyalkyl acrylates such as butoxyethyl acrylate, phenoxyethylacrylate (e.g., SR339, Sartomer Company, Inc.), and ethoxyethoxyethylacrylate; single and multi-ring cyclic aromatic or non-aromaticacrylates such as cyclohexyl acrylate, benzyl acrylate,dicyclopentadiene acrylate, dicyclopentanyl acrylate, tricyclodecanylacrylate, bornyl acrylate, isobornyl acrylate (e.g., SR423, SartomerCompany, Inc.), tetrahydrofurfuryl acrylate (e.g., SR285, SartomerCompany, Inc.), caprolactone acrylate (e.g., SR495, Sartomer Company,Inc.), and acryloylmorpholine; alcohol-based acrylates such aspolyethylene glycol monoacrylate, polypropylene glycol monoacrylate,methoxyethylene glycol acrylate, methoxypolypropylene glycol acrylate,methoxypolyethylene glycol acrylate, ethoxydiethylene glycol acrylate,and various alkoxylated alkylphenol acrylates such as ethoxylated(4)nonylphenol acrylate (e.g., Photomer 4066, IGM Resins); acrylamides suchas diacetone acrylamide, isobutoxymethyl acrylamide,N,N′-dimethyl-aminopropyl acrylamide, N,N-dimethyl acrylamide, N,Ndiethyl acrylamide, and t-octyl acrylamide; vinylic compounds such asN-vinylpyrrolidone and N-vinylcaprolactam; and acid esters such asmaleic acid ester and fumaric acid ester. With respect to the long andshort chain alkyl acrylates listed above, a short chain alkyl acrylateis an alkyl group with 6 or less carbons and a long chain alkyl acrylateis alkyl group with 7 or more carbons.

Representative polyfunctional ethylenically unsaturated monomersinclude, without limitation, alkoxylated bisphenol A diacrylates, suchas ethoxylated bisphenol A diacrylate, with the degree of alkoxylationbeing 2 or greater. The monomer component of the secondary compositionmay include ethoxylated bisphenol A diacrylate with a degree ofethoxylation ranging from 2 to about 30 (e.g. SR349 and SR601 availablefrom Sartomer Company, Inc. West Chester, Pa. and Photomer 4025 andPhotomer 4028, available from IGM Resins), or propoxylated bisphenol Adiacrylate with the degree of propoxylation being 2 or greater; forexample, ranging from 2 to about 30; methylolpropane polyacrylates withand without alkoxylation such as ethoxylated trimethylolpropanetriacrylate with the degree of ethoxylation being 3 or greater; forexample, ranging from 3 to about 30 (e.g., Photomer 4149, IGM Resins,and SR499, Sartomer Company, Inc.); propoxylated-trimethylolpropanetriacrylate with the degree of propoxylation being 3 or greater; forexample, ranging from 3 to 30 (e.g., Photomer 4072, IGM Resins andSR492, Sartomer); ditrimethylolpropane tetraacrylate (e.g., Photomer4355, IGM Resins); alkoxylated glyceryl triacrylates such aspropoxylated glyceryl triacrylate with the degree of propoxylation being3 or greater (e.g., Photomer 4096, IGM Resins and SR9020, Sartomer);erythritol polyacrylates with and without alkoxylation, such aspentaerythritol tetraacrylate (e.g., SR295, available from SartomerCompany, Inc. (West Chester, Pa.)), ethoxylated pentaerythritoltetraacrylate (e.g., SR494, Sartomer Company, Inc.), anddipentaerythritol pentaacrylate (e.g., Photomer 4399, IGM Resins, andSR399, Sartomer Company, Inc.); isocyanurate polyacrylates formed byreacting an appropriate functional isocyanurate with an acrylic acid oracryloyl chloride, such as tris-(2-hydroxyethyl) isocyanuratetriacrylate (e.g., SR368, Sartomer Company, Inc.) andtris-(2-hydroxyethyl) isocyanurate diacrylate; alcohol polyacrylateswith and without alkoxylation such as tricyclodecane dimethanoldiacrylate (e.g., CD406, Sartomer Company, Inc.) and ethoxylatedpolyethylene glycol diacrylate with the degree of ethoxylation being 2or greater; for example, ranging from about 2 to 30; epoxy acrylatesformed by adding acrylate to bisphenol A diglycidylether and the like(e.g., Photomer 3016, IGM Resins); and single and multi-ring cyclicaromatic or non-aromatic polyacrylates such as dicyclopentadienediacrylate and dicyclopentane diacrylate.

The protective coating composition may or may not include an oligomericcomponent. One or more oligomers may be present in the protectivecomposition. One class of oligomers that may be included isethylenically unsaturated oligomers. When employed, suitable oligomersmay be monofunctional oligomers, polyfunctional oligomers, or acombination of a monofunctional oligomer and a polyfunctional oligomer.If present, the oligomer component may include aliphatic and aromaticurethane (meth)acrylate oligomers, urea (meth)acrylate oligomers,polyester and polyether (meth)acrylate oligomers, acrylated acrylicoligomers, polybutadiene (meth)acrylate oligomers, polycarbonate(meth)acrylate oligomers, and melamine (meth)acrylate oligomers orcombinations thereof. The protective coating composition may be free ofurethane groups, urethane acrylate compounds, urethane oligomers, orurethane acrylate oligomers.

The oligomeric component of the protective coating composition mayinclude a difunctional oligomer. A difunctional oligomer may have astructure according to formula (I) below:

F₁—R₁-[urethane-R₂-urethane]_(m)-R₁—F₁  (I)

where F₁ may independently be a reactive functional group such asacrylate, methacrylate, acrylamide, N-vinyl amide, styrene, vinyl ether,vinyl ester, or other functional group known in the art; R₁ may include,independently, —C₂₋₁₂O—, —(C₂₋₄—O)_(n)—, —C₂₋₁₂O—(C₂₋₄—O)_(n)—,C₂₋₁₂O—(CO—C₂₋₅O)_(n)—, or —C₂₋₁₂O—(CO—C₂₋₅NH)_(n)— where n is a wholenumber from 1 to 30, including, for example, from 1 to 10; R₂ may be apolyether, polyester, polycarbonate, polyamide, polyurethane, polyurea,or combination thereof; and m is a whole number from 1 to 10, including,for example, from 1 to 5. In the structure of formula (I), the urethanemoiety may be the residue formed from the reaction of a diisocyanatewith R₂ and/or R₁. The term “independently” is used herein to indicatethat each F₁ may differ from another F₁ and the same is true for eachR₁.

The oligomer component of the curable protective composition may includea polyfunctional oligomer. The polyfunctional oligomer may have astructure according to formula (II), formula (III), or formula (IV) setforth below:

multiurethane-(F₂—R₁—F₂)_(x)  (II)

polyol-[(urethane-R₂-urethane)_(m)-R₁—F₂]_(x)  (III)

multiurethane-(R₁—F₂)_(x)  (IV)

where F₂ may independently represent from 1 to 3 functional groups suchas acrylate, methacrylate, acrylamide, N-vinyl amide, styrene, vinylether, vinyl ester, or other functional groups known in the art; R₁ caninclude —C₂₋₁₂O—, —(C₂₋₄—O)_(n)—, —C₂₋₁₂O—(C₂₋₄—O)_(n)—,—C₂₋₁₂O—(CO—C₂₋₅O)_(n)—, or —C₂₋₁₂O—(CO—C₂₋₅NH)_(n)— where n is a wholenumber from 1 to 10, including, for example, from 1 to 5; R₂ may bepolyether, polyester, polycarbonate, polyamide, polyurethane, polyureaor combinations thereof; x is a whole number from 1 to 10, including,for example, from 2 to 5; and m is a whole number from 1 to 10,including, for example, from 1 to 5. In the structure of formula (II),the multiurethane group may be the residue formed from reaction of amultiisocyanate with R₂. Similarly, the urethane group in the structureof formula (III) may be the reaction product formed following bonding ofa diisocyanate to R₂ and/or R₁.

Urethane oligomers may be prepared by reacting an aliphatic or aromaticdiisocyanate with a dihydric polyether or polyester, most typically apolyoxyalkylene glycol such as a polyethylene glycol. Moisture-resistantoligomers may be synthesized in an analogous manner, except that polarpolyethers or polyester glycols are avoided in favor of predominantlysaturated and predominantly nonpolar aliphatic diols. These diols mayinclude alkane or alkylene diols of from about 2-250 carbon atoms thatmay be substantially free of ether or ester groups.

Polyurea elements may be incorporated in oligomers prepared by thesemethods, for example, by substituting diamines or polyamines for diolsor polyols in the course of synthesis.

The protective coating composition may also contain a polymerizationinitiator to facilitate polymerization (curing) after application of theprotective coating composition to the cladding. For many acrylate-basedcoating formulations, photoinitiators, such as the known ketonicphotoinitiating and/or phosphine oxide additives, may be used. Theamount of photoinitiator may be adjusted to promote radiation cure toprovide reasonable cure speed without causing premature gelation of theprotective coating composition. A desirable cure speed may be a speedsufficient to cause curing of the coating composition of greater thanabout 90%, or greater than 95%). As measured in a dose versus moduluscurve, a cure speed for coating thicknesses of about 75 μm may be, forexample, less than 1.0 J/cm² or less than 0.5 J/cm².

Suitable photoinitiators for the protective coating composition mayinclude, without limitation, 2,4,6-trimethylbenzoyl-diphenylphosphineoxide (e.g. Lucirin TPO); 1-hydroxycyclohexylphenyl ketone (e.g.Irgacure 184 available from BASF);(2,6-diethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide (e.g. incommercial blends Irgacure 1800, 1850, and 1700, BASF);2,2-dimethoxyl-2-phenyl acetophenone (e.g., Irgacure, 651, BASF);bis(2,4,6-trimethylbenzoyl)phenyl phosphine oxide (e.g., Irgacure 819,BASF); (2,4,6-triiethylbenzoyl)diphenyl phosphine oxide (e.g., incommercial blend Darocur 4265, BASF);2-hydroxy-2-methyl-1-phenylpropane-1-one (e.g., in commercial blendDarocur 4265, BASF) and combinations thereof.

In addition to the above-described components, the protective coatingcomposition of the present invention may optionally include an additiveor a combination of additives. Representative additives include, withoutlimitation, antioxidants, catalysts, lubricants, low molecular weightnon-crosslinking resins, adhesion promoters, and stabilizers. Additivesmay operate to control the polymerization process, thereby affecting thephysical properties (e.g., modulus, glass transition temperature) of thepolymerization product formed from the composition. Additives may affectthe integrity of the polymerization product of the composition (e.g.,protect against de-polymerization or oxidative degradation).

The protective coating composition may include thiodiethylenebis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate (e.g., Irganox 1035,available from BASF) as an antioxidant. The protective coatingcomposition may include an acrylated acid adhesion promoter (such asEbecryl 170 (available from UCB Radcure (Smyrna Ga.)).

The protective coating has a higher modulus than the cladding polymermaterial. The protective coating has a Young's modulus, when configuredas a cured rod having a diameter of about 0.022″ of at least about 1000MPa, or at least about 1200 MPa, or at least about 1400 MPa, or at leastabout 1600 MPa, or at least about 1800 MPa.

The thickness of the protective coating is in the range from 30 μm to 90μm, or in the range from 40 μm to 80 μm, or in the range from 50 μm to70 μm. An optional white ink layer may be applied on top of theprotective coating. The function of the ink layer is to make sure thatthe scattered radiation is uniform in all directions. The white inklayer may include TiO₂ powder loaded into a coating material of the typeused for the protective coating. Alternatively, TiO₂ powder may beloaded directly in the protective coating.

The protective coating is formed by applying the protective coatingcomposition to the cladding and initiating reaction of the protectivecoating composition to form the protective coating on the cladding. Inone embodiment, the protective coating is formed by curing a protectivecoating composition during draw of a glass preform. When the cladding isa polymer formed by curing a cladding composition, the protectivecoating composition may be applied to the cladding composition before orafter curing of the cladding composition (wet-on-wet or wet-on-dryconfigurations).

After initial processing, the fabricated element may have a cylindricalor rod configuration. Post-fabrication processing may be employed toalter the shape of the light-diffusing element. Conventional softening,bending, and/or casting techniques may be employed to achieve bent orarbitrarily-shaped light-diffusing elements having voids in accordancewith the present disclosure, for example, can be made.

The light-diffusing element may be incorporated in an optical systemthat includes a light source. The light source may be a lamp, diode,laser, laser diode, LED (light-emitting diode) or other source. Thelight source may operate over all or part of the spectral range from 200nm to 2000 nm. To insure efficient coupling of the light source to thelight-diffusing element, it is desirable to insure that the numericalaperture of the light-diffusing element is comparable to or exceeds thenumerical aperture of the light source. The numerical aperture of thelight-diffusing element is controlled by the relative refractive indicesof the core and cladding. The numerical aperture of the light-diffusingelement can be increased by increasing the refractive index of the coreand/or decreasing the refractive index of the cladding. The refractiveindex of the core can be increased by including updopant(s) asscattering center(s) in the core glass composition. The refractive indexof the cladding can be reduced by incorporating internal voids asscattering centers in the cladding, employing polymer cladding insteadof silica glass cladding, and using fluorinated versions of claddingpolymer materials. The numerical aperture of the light-diffusing elementmay be at least 0.4, or at least 0.5, or at least 0.6, or at least 0.7,or at least 0.8.

FIG. 5 illustrates illumination systems incorporating a light-diffusingelement. Illumination system 120 includes light source 125 andlight-diffusing element 130. Light source 125 launches source light 135into light-diffusing element 130. Internal voids within light-diffusingelement 130 scatter source light 135 to produce illumination light 140that exits through the outer surface of light-diffusing element 130.Light source 125 may be in direct contact with light-diffusing element130 (e.g. “butt coupled”) or an air gap may be present. Althoughillumination light 140 is depicted for reasons of convenience ofillustration as parallel rays, it is to be understood that illuminationlight 140 may include rays of light that are directed in randomdirections.

Illumination system 150 includes light source 155, optic 160, andlight-diffusing element 165. Light source 155 launches source light 170into optic 160, which may process source light 170 to provide sourcelight 172 to light-diffusing element 165. Internal voids withinlight-diffusing element 165 scatter source light 172 to produceillumination light 175 that exits through the outer surface oflight-diffusing element 165. Although illumination light 175 is depictedfor reasons of convenience of illustration as parallel rays, it is to beunderstood that illumination light 140 may include rays of light thatare directed in random directions.

Light sources that may be incorporated in an illumination system includelamps, lasers, diodes, laser diodes, and light-emitting diodes. Anintervening optic, such as optic 160 shown in FIG. 5, is an element ofthe illumination system that may facilitate coupling of a light sourceto a light-diffusing element. The optic may collect, collimate, focus,and/or otherwise process light supplied from a light source. The opticmay be a solid glass element, a solid polymer or plastic element, aglass or polymer optical fiber, a lens or other coupling element.

Light-diffusing elements in accordance with the present disclosure maybe deployed in illumination systems, as light sources (e.g. forphotochemical reactions, cooling spaces, heating spaces, or closedspaces with controlled environments), and as luminaires. Thelight-diffusing elements are suitable for functional and decorativelighting applications.

FIG. 6 shows an image of a portion of an illustrative light-diffusingelement. Light-diffusing element 200 includes glass core 210, polymercladding 220, and protective coating 230. Glass core 210 is made fromsilica glass and has a diameter of 125 μm. Polymer cladding 220 is a lowmodulus urethane acrylate polymer having a thickness of 32.5 μm.Protective coating 230 is a high modulus urethane acrylate polymerhaving a thickness of 32.5 μm. Light-diffusing element 200 includes aplurality of scattering centers 240 in polymer cladding 220. Scatteringcenters 240 are internal voids of various sizes filled with N₂ gas andwere made by exposing the cladding composition to N₂ gas duringapplication of the cladding composition to glass core 210 and subsequentcuring of the cladding composition to form polymer cladding 220. Glasscore 210 and protective coating 230 lack scattering centers.

To assess scattering loss, a series of calculations was performed. Inthe calculation, attenuation due to light scattering from a bulkmaterial containing scattering centers was determined. The light had awavelength of 560 nm. The bulk material was a fluorinated acrylatepolymer having a refractive index of 1.37 at 560 nm. The fluorinatedacrylate polymer is representative of cladding polymer materials. Thescattering centers included internal voids in the form of spherical gasbubbles having a refractive index of 1.0 and TiO₂ nanoparticles having arefractive index of 2.4. In a first calculation, the scattering centerswere spherical gas bubbles having a diameter of 1 μm and a concentrationof 100/mm³ in the bulk material. The scattering loss was calculated tobe 2 dB/m. In a second calculation, the scattering centers werespherical gas bubbles having a diameter of 5 μm and a concentration of10/mm³ in the bulk material. The scattering loss was calculated to be 3dB/m. In a third calculation, the scattering centers were spherical gasbubbles having a diameter of 10 μm and a concentration of 1/mm³ in thebulk material. The scattering loss was calculated to be 1.6 dB/m. In afourth calculation, the scattering centers were TiO₂ nanoparticleshaving a diameter of 0.2 μm and a concentration of 2000/mm³ in the bulkmaterial. The scattering loss was calculated to be 2.4 dB/m.

FIG. 7 shows the results of a separate calculation that showsattenuation losses as a function of radius for gas-filled internal voidand TiO₂ scattering centers in the fluorinated acrylate polymer bulkmaterial. The calculation for FIG. 7 assumes a constant concentration of1 scattering center per cubic micron. The dimensions of attenuation arem⁻¹, which can be converted to dB/m by multiplying by 10. Data pointsfor radius values less than 1 μm correspond to TiO₂ scattering centers.Data points for radius values greater than 1 μm correspond to gas-filledinternal void scattering centers. Data points for each type ofscattering center are given at a radius value of 1 μm. The results showan increase in attenuation (scattering loss) as the radius of thescattering center increases. As indicated hereinabove, the presence ofgas-filled internal voids in the cladding lowers the average refractiveindex of the cladding and is a recommended strategy for increasing thenumerical aperture of light-diffusing elements to achieve bettercoupling efficiency with light sources having high numerical apertures.The presence of TiO₂ nanoparticles, in contrast, leads to an increase inthe average refractive index of the cladding and makes it more difficultto obtain a light-diffusing element having a high numerical aperture.

Although the calculations described herein refer to bulk claddingpolymer material instead of cladding polymer material in theconfiguration of a the present light-diffusing elements, the results arepredictive of the scattering losses expected for light-diffusingelements having glass cores without scattering centers and a fluorinatedacrylate polymer with gas-filled internal voids or TiO₂ nanoparticles asscattering centers. In a typical light-diffusing element, most of thelight is guided by the core and approximately 2-3% of the light entersthe cladding as an evanescent field. The scattering losses calculatedfor the bulk material can be accordingly scaled to provide estimates ofthe scattering loss available from the bulk material in theconfiguration of a polymer cladding as described herein.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the illustrated embodiments. Since modifications,combinations, sub-combinations and variations of the disclosedembodiments that incorporate the spirit and substance of the illustratedembodiments may occur to persons skilled in the art, the descriptionshould be construed to include everything within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A light-diffusing element comprising: a glasscore, said glass core having a diameter greater than 65 μm; and acladding surrounding said glass core, said cladding having a lowerrefractive index than said glass core, said cladding including firstscattering centers, said first scattering centers having a cross-sectionwith a dimension of at least 25 nm; wherein said element exhibits lightscattering losses of at least 0.1 dB/m.
 2. The light-diffusing elementof claim 1, wherein said cladding comprises glass.
 3. Thelight-diffusing element of claim 2, wherein said first scatteringcenters include internal voids, said internal voids being filled by agas and having a cross-section with a dimension between 50 nm and 20 μm.4. The light-diffusing element of claim 1, wherein said claddingcomprises a polymer.
 5. The light-diffusing element of claim 4, whereinsaid first scattering centers include internal voids, said internalvoids being filled by a gas and having a cross-section with a dimensionbetween 25 nm and 40 μm.
 6. The light-diffusing element of claim 5,wherein said gas is selected from the group consisting of N₂, air, andAr.
 7. The light-diffusing element of claim 5, wherein said internalvoids have a cross-section with a dimension between 50 nm and 20 μm. 8.The light-diffusing element of claim 5, wherein the concentration ofsaid internal voids in said cladding is at least 1.0% by volume.
 9. Thelight-diffusing element of claim 4, wherein said first scatteringcenters include nanoparticles, said nanoparticles having a cross-sectionwith a dimension of at least 25 nm.
 10. The light-diffusing element ofclaim 9, wherein said cross-sectional dimension of said nanoparticles isless than 500 nm.
 11. The light-diffusing element of claim 10, whereinsaid nanoparticles have a composition selected from the group consistingof Al₂O₃, SiO₂, ZrO₂, Y₃Al₅O₁₂, and rare earth oxides.
 12. Thelight-diffusing element of claim 10, wherein said nanoparticles areluminescent.
 13. The light-diffusing element of claim 10, wherein saidnanoparticles have a concentration in said cladding of at least 1.0% byvolume.
 14. An illumination system comprising: a light source opticallycoupled to a light-diffusing element, said light-diffusing elementcomprising: a glass core, said glass core having a diameter greater than65 μm; and a cladding surrounding said glass core, said cladding havinga lower refractive index than said glass core, said cladding includingfirst scattering centers, said first scattering centers having across-section with a dimension of at least 25 nm; wherein said elementexhibits light scattering losses of at least 0.1 dB/m
 15. Theillumination system of claim 14, wherein said light source is an LED ora laser diode.
 16. A method for forming a light-diffusing elementcomprising: forming a core, said core comprising glass; forming acladding on said core, said cladding including first scattering centers,said first scattering centers having a cross-section with a dimension ofat least 25 nm.
 17. The method of claim 16, wherein said forming coreincludes drawing a glass preform.
 18. The method of claim 17, whereinsaid forming cladding includes applying a curable composition to saidcore and curing said curable composition.
 19. The method of claim 18,wherein said forming cladding includes curing said curable compositionin the presence of a void-producing gas.
 20. The method of claim 18,wherein said first scattering centers are nanoparticles, and whereinsaid curable composition further includes said nanoparticles.